Ultrasonic welding allows accurate and precise application of energy to selectively melt or weld the desired portions of a plastic assembly. Ultrasonic welding is suitable for most thermoplastic materials, and is widely used in the automotive, packaging, electronic, and consumer industries. In practice, high-frequency (ultrasonic) mechanical vibrations are transmitted by the ultrasonic welding machine to mating plastic parts. At the joint or interface of the two parts, a combination of applied force and surface and/or intermolecular friction increases the temperature until the melting point of the thermoplastic is reached. The ultrasonic energy is then removed and a molecular bond or weld is produced between the two plastic parts.
An ultrasonic welding system typically contains a high-frequency power supply (usually 20-40kHz). The high-frequency energy is directed into a horn which is a bar or a metal section, typically of titanium, aluminum, or hardened steel, dimensioned to be resonant at the applied frequency. The horn contacts the workpiece and transmits the mechanical vibrations into it. A fixture or nest supports and aligns the two parts to be welded. Proper joint design is essential for optimum welding results. Factors such as the type of material, part geometry, and requirements of the welded joint must be considered when determining joint design. A joint should have some means of alignment and a small, uniform initial contact area to concentrate the ultrasonic energy for rapid localized energy dissipation. An energy director, the most commonly used design, consists of a small triangular bead of material on the part surface to be welded. During welding, the interfaces melt and telescope together, producing a weld in the shear mode.
Many types of thermoplastic polymers can be welded. Amorphous resins such as polystyrene, acrylonitrile-butadiene-styrene (ABS), polycarbonate, etc., are very energy efficient and are generally preferred for ultrasonic welding. They are characterized by a random molecular arrangement and a broad softening temperature range. This allows the material to flow easily without premature solidification. Resins that have higher levels of crystallinity require higher ultrasonic energy levels because of their highly-ordered molecular structure. Some dissimilar resins can be welded together if their glass transition temperatures are similar (typically within about 40.degree. F.), such as ABS-to-acrylic, polycarbonate-to-acrylic, and polystyrene-to-polyphenylene oxide.
The conventional wisdom in the art of ultrasonic welding has restricted use to attachment of one plastic piece to another plastic piece. Situations that require several pieces to be joined require two or more separate operations. Clearly it would be an advantageous improvement in the state of the art to be able to weld more than two plastic parts together in a single step.